1. Field of the Invention
This invention relates in general to electron spectroscopic analysis of material surfaces and in particular to the electron spectroscopic analysis of the surfaces of electrically insulating materials by illuminating specimens to cause the emission of electrons which may be analyzed to determine the characteristics of the surfaces.
2. Brief Description of the Prior Art
It is known that the surface analysis of insulating materials by electron or ion spectroscopy is hampered by the buildup of positive charges on the surface. For example, in X-Ray Photoelectron Spectroscopy (XPS), a beam of x-rays illuminates a portion of a specimen, causing electrons to be emitted. These emissions are analyzed to determine the composition of the surface.
The emission of electrons from the surface leaves the surface positively charged. The positive charge causes the energy of the emitted electrons to change and makes the interpretation of their energy spectra difficult. In addition, if the x-ray beam is focused on the specimen so that its intensity varies from one location to another, the spectrum of the electrons is smeared out in energy and much of the information contained in the spectrum is lost.
A number of solutions to this problem are currently used in commercial instrumentation, most of which involve illuminating the sample with a beam of low energy electrons to neutralize the positive charges on the specimen surface. In many cases such solutions are acceptable, but in the case of very non-uniform x-ray beams such as those found in monochromatized instruments, the neutralization is not good enough to analyze many materials. Additional solutions have been developed such as the positioning of an electrically conductive grid close to the surface of the sample to smooth gradients in the electrical potential in the region of the x-ray beam spot, but these work only in a limited number of cases.
In a typical XPS spectrometer, a nonuniform beam of x-rays illuminates a specimen, and the photoemitted electrons are energy analyzed to determine the elements and chemical compounds present in the specimen. If the specimen is insulating, the surface potential becomes increasingly positive as electrons leave, until it becomes positive enough to attract other electrons present in the ambient environment. As a consequence, the surface potential becomes nonuniform, with areas receiving the most x-rays being the most positive. A typical electron beam neutralizer is an electron gun, usually a simple heated tungsten filament with an electrostatic lens. The gun floods the specimen surface with electrons, thereby charging that surface to approximately the energy of the electrons, at which point the electrons can no longer energetically reach the specimen. Where the x-rays illuminate the specimen, a dynamic equilibrium is established, in which the surface potential is that value for which the electron fluxes to and from the specimen are equal.
However, the portion of the specimen which is not illuminated by the x-ray beam will charge to the potential of the most energetic electrons in the environment, since without the x-rays, there is no mechanism for an electron, once reaching the surface, to leave. This creates a situation in which electrons from the flooding beam, trying to reach the region which the x-rays are striking to cancel the positive surface charge, are repelled by the surrounding surface which is negatively charged. As a consequence, a considerably nonuniform potential develops on the specimen surface. Attempts to focus the electron beam to the area illuminated by the x-rays are not effective, because the electrons are defocused as they are repelled by the specimen.